1. Field of the Invention
The invention relates generally to the lubrication of gearboxes and more particularly to systems for improving lubrication in relation to large wind turbine gearboxes having superfinished components.
2. Description of the Related Art
Engineering components in moving contact with one another generally require lubrication in order to prevent or reduce friction, heat and wear. The greater the load, speed or period of movement, the more critical is the lubrication. Gears, bearings and cams are all examples of engineering components that may be designed for thousands of hours of continuous operation. Only with correct lubrication can such operation be successfully achieved. Various principles of lubrication exist, including air bearings, hydrodynamic lubrication and particulate lubrication. The present invention is concerned with liquid lubrication in the form of oils, both natural and synthetic.
Complex engineering components often operate as part of a system, such as an engine, gearbox or differential. These systems may be provided with lubrication that fulfils all of the requirements of the whole system. Often this will entail providing a supply of lubricant that circulates through the system. The lubricant must be carefully chosen in order to provide adequate lubrication to the different components. Often this choice is a compromise as one component may require one form of lubricant while a different type of lubricant may be more suited to another component. The lubricant must also be chosen with particular operating conditions in mind. Operation of the system outside the preferred range of load, speed or temperature may lead to less than adequate lubrication. Inadequate lubrication may lead to boundary lubrication conditions, contact fatigue and eventually rough cleavage of peak asperities (micropitting) from the metal surface. Cleaved metal asperities, or particles and especially particles larger than 1 micron contaminate the lubricant and accelerate the wear process. In particular, such particles may be extremely detrimental to the bearings which generally operate with a lower film thickness to that of the gears.
Consequently, gear life is shortened and premature catastrophic gear failure may occur. In an attempt to make lubricants more suited to a range of conditions, additive packages may be added to a base oil to improve its performance. These additives may however themselves be sensitive to certain conditions and can become ineffective or even detrimental. They are also often environmentally undesirable.
One factor that determines the lubrication regime of a given component is the surface roughness of the contact surfaces. Recent advances in finishing techniques have allowed engineers to achieve smoother surfaces than were previously possible. An additional factor is the nature of the finished surface. Ground or honed surfaces may have a relatively symmetrical roughness profile in which peaks and troughs are present in equal numbers. Other polishing and finishing techniques may achieve a planarized surface in which primarily the peak asperities have been removed. Planarized surfaces may be characterized by a material ratio (Rmr) of greater than 50% and are generally recognized as having better load bearing capacity than a symmetrical surface of similar roughness. In particular, superfinishing techniques are now available that are capable of providing mirror-like smoothness even to relatively large and complex components. In the following, the terms “superfinishing” and “superfinished surfaces” will be used to denote surface finishes with a roughness (Ra) of 0.25 microns or less, irrespective of the manner in which the surface finish is achieved. However, it must be noted that Ra cannot distinguish between a surface that is planarized versus a surface that is ground or honed. Those trained in the art will understand that alternative tribological parameters such as Rpm (mean peak height) may better distinguish a planarized surface from a ground or honed surface. Moreover, mathematical descriptions such as the lambda ratio may be defined using those alternative parameters.
Various theories exist regarding the correct lubrication for use with superfinished components but in general, such surfaces operate outside the accepted ranges where tried and trusted traditional results apply. According to US Patent Publication No. 2009/0151494, it may be advantageous to apply a higher viscosity lubricant to superfinished surfaces in order to improve their high-temperature operation without sacrificing lubricant film strength. In a further US Patent Publication No. 2009/0137436, a complex additive package is disclosed that may be used to improve load carrying capacity and enhance surface fatigue life of mechanical components, including those that have superfinished surfaces.
An additional function that a lubricant may perform is the removal of debris that may be formed by wearing of surfaces or as a consequence of foreign objects. Circulation of the lubricant through a filter allows such debris to be removed, preventing further damage to the system. In theory, it would be desirable to have a filter capable of removing all particles sufficiently large to cause damage to the system. Particles having a size greater than the operational lubricant film thickness may be considered potentially harmful. Nevertheless, it is not always simple to remove them from the system since the finer the filter, the more difficult it is to circulate a lubricant through it. The lubricant film thickness will also be at least partially dependent upon the oil viscosity. As the viscosity increases, the system may become tolerant of larger debris but the filtering of such debris becomes more difficult due to the resistance of the filter to flow.
Additionally, parasitic frictional losses increase as a result of greater lubricant viscosity. These in turn generate heat which causes the system to run hotter or require additional oil cooling capacity. Additional additives may be required specifically to compensate for such higher temperatures or otherwise offset the consequences of high viscosity, in particular under cold-start conditions. In general, the lifetime of a lubricant is strongly dependent upon temperature. Exposure to elevated temperatures can result in lubricant deterioration due to e.g. thermal oxidation.
Lubricants circulating in a lubrication system also tend to entrain air. Air bubbles or inclusions are generally undesirable both due to the tendency to cause foaming but also because the presence of air inclusions can reduce the strength of the lubricant film. Higher viscosity lubricants have greater tendency to entrain air than those of lower viscosity. In this context too, further additives may be provided to the additive package in an effort to counteract the effect of air bubbles or to prevent them from occurring.
One particular area of operation is the field of wind turbine gearboxes. These systems are becoming ever bigger and turbines presently being installed may typically be rated at more than 1 MW. A significant characteristic of such machines is that the gearbox input stage operates at very low speeds (rotor speeds between 5-10 RPM). This corresponds to input stage sun pinion speeds of 30-60 RPM in a typical 1.5 MW epicyclic gear input stage with a gear ratio of approximately 6.14. While the high speed output stage may rotate at 1500 RPM or above. The gears and bearings are highly loaded and operate under extremely harsh conditions that are unique to the wind power industry. The lubricant package plays a critical role in ensuring that these gearboxes have a service life of more than twenty years. The volume of lubricants used in each gearbox is greater than 200 liters and unlike the short interval of oil changes in a car, wind turbines must operate for 25,000 to 50,000 hours between lubricant changes. Changing the lubricant is also no easy chore since the gearboxes are located atop towers that can extend more than one hundred meters above the ground often located in harsh climates and remote regions on high terrain or offshore.
The presently approved lubricant viscosity for most large wind turbines is at least ISO viscosity grade (ISO VG) 320. The ISO VG scale is a globally recognized standard for kinematic viscosity in units of mm2/s, measured at 40° C. Henceforth, all references to viscosity are made on the ISO VG basis. The approved viscosity is a significant compromise, since it may in fact be too low for the low speed input stage and is generally considered too high for high speed gear stages and especially the bearings. The high speed bearings of 1.5 megawatt turbines and larger operate under very tight tolerances. High viscosity lubricants restrict proper bearing rolling and promote frictional heating and wear. It would also be desirable to filter particulates down to below 3 microns. Nevertheless, for high viscosity lubricants, filters of such fineness are often ineffective and once initial run-in has been carried out may be replaced by 10 micron filters. The American Gear Manufacturers Association publication ANSI/AGMA/AWEA 6006-A03 “Standard for Design and Specification of Gearboxes for Wind Turbines” specifies a 10 micron inline filter with automatic bypass for plugged filter and cold start conditions. Additional, finer filters of 3 microns may be used in an offline configuration but these cannot guarantee full removal of all particulates.
The presently recommended wind turbine gearbox lubricants are composed of base oil and a relatively high concentration of additives. The base oil can be mineral oil based or synthetic. The additives may be provided to improve shear and thermal oxidation stability and to prevent sludge deposits, reduce wear, micropitting, scuffing, foaming, corrosion and bacterial growth. In particular anti-wear, anti-micropitting and anti-scuffing additives may be added in order to compensate the insufficient viscosity for the low-speed stage. Formulating such a lubricant is a daunting task given the multitude of additives that are added to accomplish the above objectives. The formulator must be knowledgeable of the additive chemistry, the chemical interaction between the various additives, and their reaction with the various copper and steel alloys present in the gearbox. The formulation of lubricants is a balancing act, since certain additives are reactive with the metal surfaces, oxygen, and even the other additives. Zinc alkyldithiophosphate additives (ZDDP) are well known examples of compounds that react negatively with certain metal surfaces and can promote micropitting.
By their nature the additives used are not always environmentally friendly and leaks to the environment would be damaging to the green image of the wind power industry. In particular, sulfur and phosphorous are damaging to the environment. Additionally sulfur and phosphorous containing additives may damage gearboxes by promoting micropitting and corrosion. Scientists, engineers and formulators continuously search for compounds that improve antiwear, anti-micropitting, anti-scuffing, extreme pressure, and anti-oxidant properties of commercial lubricants while reducing or even eliminating phosphorous and sulfur containing compounds. However this often leads to even more complex additives using additional compounds and processes.
For example, U.S. Pat. No. 7,759,294 B2 mentions that the use of hydrocarbylamine with alkylphosphorothioate enhances the “load carrying capacity” capabilities of the lubricant. Furthermore, U.S. Pat. No. 7,612,025 B2 mentions that the use of an alkali or alkaline earth metal salicylate based and/or overbased salt, up to 0.15 percent boron by mass and reduced sulfur content of 0.3 percent or less by mass in conjunction with a phosphorous containing antiwear agent such as ZDDP improves the antiwear properties of the lubricant.
Further US Patent Application 2006/0276355 A1 claims that mixing two lubricants one of viscosity preferably less than 10 cSt and one preferably greater than 100 cSt and having common antiwear, anti-micropitting, anti-scuffing, extreme pressure, and anti-oxidant, anti-corrosion and anti-foaming additives decreases micropitting. According to the example given, the lubricants are formulated to ISO VG 320 as recommended by wind turbine gearbox manufacturers.
It would be desirable to eliminate or reduce the concentration of certain additives to create a simpler and more environmentally friendly lubricant. It would also be desirable to operate gearboxes with cleaner oil having a lower upper size limit for particulate debris.